Ammonia synthesis is one of the largest industrially-practiced chemical reactions due to the widespread use of ammonia. For example, ammonia is used in the production of fertilizers, explosives, fibers, plastics and pharmaceuticals, and as a refrigerant in large scale refrigeration plants and in air conditioning systems for buildings of all kinds. Ammonia is also used in the pulp and paper industry, in mining and metallurgy, and as a cleaning agent.
Industrially, ammonia is produced by the Haber process, which was developed at the beginning of the twentieth century. The Haber process involves reacting gaseous nitrogen and gaseous hydrogen over an iron-based catalyst at high temperature and pressure. However, the yield of ammonia by the Haber process is limited by the effect of temperature on the reaction equilibrium. Specifically, decreasing the temperature of the reaction causes the equilibrium position to shift toward the formation of ammonia, resulting in a higher yield of ammonia. However, as the rate of reaction at lower temperatures is extremely slow, a higher temperature is used to obtain practical reaction rates, which reduces the amount of the yield of ammonia, i.e. typically to a level of about 10-20%. In the industrial production of ammonia, the gas mixture from the reactor is cooled to liquefy the ammonia and the remaining mixture of reactant gases is recycled back to the reactor to obtain higher overall yields.
Ionic liquids have been shown to absorb large amounts of ammonia (Yokozeki et al, Ind. Eng. Chem. Res., 46:1605-1610, 2007; and Yokozeki et al, Appl. Energy, 84:1258-1273, 2007); and the use of ionic liquids as a working fluid with various refrigerants, including ammonia, in absorption refrigeration cycles is described by Shiflett et al (U.S. Patent Application Publication No. 2006/0197053). A need nevertheless remains for an improved process to produce ammonia in higher yields.